This invention concerns a method of increasing retention and drainage in papermaking using high molecular weight water-soluble anionic or nonionic dispersion polymers.
In the manufacture of paper, a papermaking furnish is formed into a paper sheet. The papermaking furnish is an aqueous slurry of cellulosic fiber having a fiber content of about 4 weight percent (percent dry weight of solids in the furnish) or less, and generally around 1.5% or less, and often below 1.0% ahead of the paper machine, while the finished sheet typically has less than 6 weight percent water. Hence the dewatering and retention aspects of papermaking are extremely important to the efficiency and cost of the manufacture.
Gravity dewatering is the preferred method of drainage because of its relatively low cost. After gravity drainage more expensive methods are used for dewatering, for instance vacuum, pressing, felt blanket blotting and pressing, evaporation and the like. In actual practice a combination of such methods is employed to dewater, or dry, the sheet to the desired water content. Since gravity drainage is both the first dewatering method employed and the least expensive, an improvement in the efficiency of this drainage process will decrease the amount of water required to be removed by other methods and hence improve the overall efficiency of dewatering and reduce the cost thereof.
Another aspect of papermaking that is extremely important to the efficiency and cost is retention of furnish components on and within the fiber mat. The papermaking furnish represents a system containing significant amounts of small particles stabilized by colloidal forces. The papermaking furnish generally contains, in addition to cellulosic fibers, particles ranging in size from about 5 to about 1000 nm consisting of, for example, cellulosic fines, mineral fillers (employed to increase opacity, brightness and other paper characteristics) and other small particles that generally, without the inclusion of one or more retention aids, would in significant portion pass through the spaces (pores) between the mat formed by the cellulosic fibers on the papermachine.
Greater retention of fines, fillers, and other components of the furnish permits, for a given grade of paper, a reduction in the cellulosic fiber content of such paper. As pulps of lower quality are employed to reduce papermaking costs, the retention aspect of papermaking becomes more important because the fines content of such lower quality pulps is generally greater. Greater retention also decreases the amount of such substances lost to the whitewater and hence reduces the amount of material costs, the cost of waste disposal and the adverse environmental effects therefrom. It is generally desirable to reduce the amount of material employed in a papermaking process for a given purpose, without diminishing the result sought. Such add-on reductions may realize both a material cost savings and handling and processing benefits.
Another important characteristic of a given papermaking process is the formation of the paper sheet produced. Formation may be determined by the variance in light transmission within a paper sheet, and a high variance is indicative of poor formation. As retention increases to a high level, for instance a retention level of 80 or 90%, the formation parameter generally declines.
Various chemical additives have been utilized in an attempt to increase the rate at which water drains from the formed sheet, and to increase the amount of fines and filler retained on the sheet. The use of high molecular weight water-soluble polymers is a significant improvement in the manufacture of paper. These high molecular weight polymers act as flocculants, forming large flocs which deposit on the sheet. They also aid in the dewatering of the sheet. In order to be effective, conventional single and dual polymer retention and drainage programs require incorporation of a higher molecular weight component as part of the program. In these conventional programs, the high molecular weight component is added after a high shear point in the stock flow system leading up to the headbox of the paper machine. This is necessary since flocs are formed primarily by a bridging mechanism and their breakdown is a largely irreversible process. For this reason, most of the retention and drainage performance of a flocculant is lost by feeding it before a high shear point. To their detriment, feeding high molecular weight polymers after the high shear point often leads to formation problems. The feed requirements of the high molecular weight polymers and copolymers which provide improved retention often lead to a compromise between retention and formation.
While successful, high molecular weight flocculant programs are improved by the addition of so called inorganic xe2x80x9cmicroparticlesxe2x80x9d. One such program employed to provide an improved combination of retention and dewatering is described in U.S. Pat. Nos. 4,753,710 and 4,913,775 incorporated herein by reference, in which a high molecular weight linear cationic polymer is added to the aqueous cellulosic papermaking suspension before shear is applied to the suspension, followed by the addition of bentonite after the shear application. Shearing is generally provided by one or more of the cleaning, mixing and pumping stages of the papermaking process, and the shear breaks down the large flocs formed by the high molecular weight polymer into microflocs. Further agglomeration then ensues with the addition of the bentonite clay particles.
Although, as described above, the microparticle is typically added to the furnish after the flocculant and after at least one shear zone, the microparticle effect can also be observed if the microparticle is added before the flocculant and the shear zone (U.S. Pat. No. 4,305,781).
Another program where an additive is injected prior to the flocculant is the so-called xe2x80x9cenhancer/flocculantxe2x80x9d treatment. Enhancer programs are comprised of the addition of an enhancer, such as phenolformaldehyde resin, to the furnish, followed by addition of a high molecular weight, nonionic flocculant such as polyethylene oxide (U.S. Pat. No. 4,070,236). In such systems, the enhancer improves the performance of the flocculant.
In a single polymer/microparticle retention and drainage aid program, a flocculant, typically a cationic polymer, is the only polymer material added along with the microparticle. Another method of improving the flocculation of cellulosic fines, mineral fillers and other furnish components on the fiber mat using a microparticle is in combination with a dual polymer program which uses, in addition to the microparticle, a coagulant and flocculant system. In such a system a coagulant is first added, for instance a low molecular weight synthetic cationic polymer or cationic starch. The coagulant may also be an inorganic coagulant such as alum or polyaluminum chlorides. This addition can take place at one or several points within the furnish make up system, including but not limited to the thick stock, white water system, or thin stock of a machine. This coagulant generally reduces the negative surface charges present on the particles in the furnish, such as cellulosic fines and mineral fillers, and thereby accomplishes a degree of agglomeration of such particles. However, in the presence of other detrimental anionic species, the coagulant serves to neutralize the interfering species enabling aggregation with the subsequent addition of a flocculant. Such a flocculant generally is a high molecular weight synthetic polymer which bridges the particles and/or agglomerates, from one surface to another, binding the particles into larger agglomerates. The presence of such large agglomerates in the furnish, as the fiber mat of the paper sheet is being formed, increases retention. The agglomerates are filtered out of the water onto the fiber web, whereas unagglomerated particles would, to a great extent, pass through such a paper web. In such a program the order of addition of the microparticle and flocculant can be reversed successfully.
However, there is continuing need to develop new retention aids to increase the efficiency of pulp or paper manufacture.
Commonly assigned U.S. Pat. No. 5,605,970 discloses a process for preparing certain high-molecular weight anionic polymer dispersions. Commonly assigned U.S. Pat. No. 5,837,776 discloses certain high molecular weight anionic flocculants and a process for their preparation. A process for the production of a water-soluble polymer dispersion in the presence of a dispersant, wherein the dispersant may be a poly(2-acrylamido-2-methyl propane sulfonic acid (AMPS)) or a copolymer having 30 or more mole percent of AMPS is disclosed in EP 0 183 466.
This invention is directed to a method of increasing retention-and drainage in a papermaking furnish comprising adding to the furnish an effective flocculating amount of a high molecular weight water-soluble dispersion polymer wherein the dispersion polymer has a bulk Brookfield viscosity of from about 10 to about 25,000 cps at 25xc2x0 C. and comprises from about 5 to about 50 weight percent of a water-soluble polymer prepared by polymerizing under free radical forming conditions in an aqueous solution of a water-soluble salt in the presence of a stabilizer:
i. 0-100 mole percent of at least one anionic monomer, and,
ii. 100-0 mole percent of at least one non-ionic vinyl monomer;
wherein the stabilizer is an anionic water-soluble polymer having an intrinsic viscosity in 1M NaNO3 of from about 0.1-10 dl/g and comprises from about 0.1 to about 5 weight percent based on the total weight of the dispersion, and the water-soluble salt is selected from the group consisting of ammonium, alkali metal and alkaline earth metal halides, sulfates, and phosphates and comprises from about 5 to about 40 weight percent based on the weight of the dispersion.
xe2x80x9cMonomerxe2x80x9d means a polymerizable allylic, vinylic or acrylic compound.
xe2x80x9cAnionic monomerxe2x80x9d means a monomer as defined herein which possesses a net negative charge. Representative anionic monomers include acrylic acid, methacrylic acid, 2-acrylamido-2-methyl-1-propanesulfonic acid, acrylamidomethylbutanoic acid, maleic acid, fumaric acid, itaconic acid, vinyl sulfonic acid, styrene sulfonic acid, vinyl phosphonic acid, allyl sulfonic acid, allyl phosphonic acid, sulfomethylated acrylamide, phosphonomethylated acrylamide and the water-soluble alkali metal, alkaline earth metal, and ammonium salts thereof. The choice of anionic monomer is based upon several factors including the ability of the monomer to polymerize with the desired comonomer, the use of the produced polymer, and cost. A preferred anionic monomer is acrylic acid.
In certain instances, it may be possible to chemically modify a non-ionic monomer component contained in the dispersion polymer of the invention after polymerization to obtain an anionic functional group, for example, the modification of an incorporated acrylamide mer unit to the corresponding sulfonate or phosphonate.
xe2x80x9cNonionic monomerxe2x80x9d means a monomer as defined herein which is electrically neutral. Representative nonionic monomers include acrylamide, methacrylamide, N-methylacrylamide, N-isopropylacrylamide, N-t-butyl acrylamide, N-methylolacrylamide, N, N-dimethyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N-(2-hydroxypropyl)methacrylamide, N-methylolacrylamide, N-vinylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, poly(ethylene glycol)(meth)acrylate, poly(ethylene glycol) monomethyl ether mono(meth)acryate, N-vinyl-2-pyrrolidone, glycerol mono((meth)acrylate), 2-hydroxyethyl(meth)acrylate, vinyl methylsulfone, vinyl acetate, and the like. Preferred nonionic monomers of include acrylamide, methacrylamide, N-isopropylacrylamide, N-t-butyl acrylamide, and N-methylolacrylamide. More preferred nonionic monomers include acrylamide and methacrylamide. Acrylamide is still more preferred.
RSV stands for Reduced Specific Viscosity. Reduced Specific Viscosity is an indication of polymer chain length and average molecular weight. Polymer chain length and average molecular weight are indicative of the extent of polymerization during production. The RSV is measured at a given polymer concentration and temperature and calculated as follows:   RSV  =            [                        (                      η            /                          η              o                                )                -        1            ]        c  
xcex7=viscosity of polymer solution
xcex7o=viscosity of solvent at the same temperature
c=concentration of polymer in solution.
In this patent application, the units of concentration xe2x80x9ccxe2x80x9d are (grams/100 ml or g/deciliter). Therefore, the units of RSV are dl/g. In this patent application, for measuring RSV, the solvent used is 1.0 Molar sodium nitrate solution. The polymer concentration in this solvent is 0.045 g/dl. The RSV is measured at 30xc2x0 C. The viscosities xcex7 and xcex7o were measured using a Cannon Ubbelohde semimicro dilution viscometer, size 75. The viscometer is mounted in a perfectly vertical position in a constant temperature bath adjusted to 30xc2x10.02xc2x0 C. The error inherent in the calculation of RSV is about 2 dl/grams. When two polymers of the same composition have similar RSV""s measured under identical conditions that is an indication that they have similar molecular weights.
IV stands for intrinsic viscosity, which is RSV when the limit of polymer concentration is zero.
xe2x80x9cInverse emulsion polymerxe2x80x9d and xe2x80x9clatex polymerxe2x80x9d mean a self-inverting water in oil polymer emulsion comprising a polymer according to this invention in the aqueous phase, a hydrocarbon oil for the oil phase, a water-in-oil emulsifying agent and an inverting surfactant. Inverse emulsion polymers are hydrocarbon continuous with the water-soluble polymers dispersed as micron sized particles within the hydrocarbon matrix. The inverse emulsion polymers are then xe2x80x9cinvertedxe2x80x9d or activated for use by releasing the polymer from the particles using shear, dilution, and, generally, another surfactant.
Inverse emulsion polymers are prepared by dissolving the required monomers in the water phase, dissolving the emulsifying agent in the oil phase, emulsifying the water phase in the oil phase to prepare a water-in-oil emulsion, homogenizing the water-in-oil emulsion, polymerizing the monomers dissolved in the water phase of the water-in-oil emulsion to obtain the polymer and then adding the self-inverting surfactant to obtain the water-in-oil self-inverting water-in-oil emulsion.
xe2x80x9cDispersion polymerxe2x80x9d means a water-soluble polymer dispersed in an aqueous continuous phase containing one or more inorganic salts. In the process of dispersion polymerization, the monomer and the initiator are both soluble in the polymerization medium, but the medium is a poor solvent for the resulting polymer. Accordingly, the reaction mixture is homogeneous at the onset, and the polymerization is initiated in a homogeneous solution. Depending on the solvency of the medium for the resulting oligomers or macroradicals and macromolecules, phase separation occurs at an early stage. This leads to nucleation and the formation of primary particles called xe2x80x9cprecursorsxe2x80x9d and the precursors are colloidally stabilized by adsorption of stabilizers. The particles are believed to be swollen by the polymerization medium and/or the monomer, leading to the formation of spherical particles having a size in the region of xcx9c0.1-10.0 microns.
xe2x80x9cAnionic dispersion polymerxe2x80x9d means a dispersion polymer as defined herein which possesses a net negative charge.
xe2x80x9cNonionic dispersion polymerxe2x80x9d means a dispersion polymer as defined herein which is electrically neutral.
In any dispersion polymerization, the variables that are usually controlled are the concentrations of the stabilizer, the monomer and the initiator, solvency of the dispersion medium, and the reaction temperature. It has been found that these variables can have a significant effect on the particle size, the molecular weight of the final polymer particles, and the kinetics of the polymerization process.
Particles produced by dispersion polymerization in the absence of any stabilizer are not sufficiently stable and may coagulate after their formation. Addition of a small percentage of a suitable stabilizer to the polymerization mixture produces stable dispersion particles. Particle stabilization in dispersion polymerization is usually referred to as xe2x80x9csteric stabilizationxe2x80x9d. Good stabilizers for dispersion polymerization are polymer or oligomer compounds with low solubility in the polymerization medium and moderate affinity for the polymer particles.
As the stabilizer concentration is increased, the particle size decreases, which implies that the number of nuclei formed increases with increasing stabilizer concentration. The coagulation nucleation theory very well accounts for the observed dependence of the particle size on stabilizer concentration, since the greater the concentration of the stabilizer adsorbed the slower will be the coagulation step. This results in more precursors becoming mature particles, thus reducing the size of particles produced.
As the solvency of the dispersion medium increases, (a) the oligomers will grow to a larger MW before they become a precursor nuclei, (b) the anchoring of the stabilizer moiety will probably be reduced and (c) the particle size increases. As the initiator concentration is increased, it has been observed that the final particle size increases. As for the kinetics, it is reported that when the dispersion medium is a non-solvent for the polymer being formed, then the locus of polymerization is largely within the growing particles and the system follows the bulk polymerization kinetics, n (the kinetic chain length)=Rp/Rt, where Rp is the propagation rate and Rt is the termination rate. As the solvency of the dispersion medium for the growing polymer particle is increased, polymer growth proceeds in solution. The polymeric radicals that are formed in solution are then captured by growing particles. Consequently, the locus of the particle polymerization process changes and there is a concomitant change in the kinetics of polymerization.
The dispersion polymers of the instant invention contain from about 0.1 to about 5 weight percent based on the total weight of the dispersion of a stabilizer.
Stablizers as used herein include anionically charged water-soluble polymers having a molecular weight of from about 100,000 to about 5,000,000 and preferably from about 1,000,000 to about 3,000,000. The stabilizer polymer must be soluble or slightly soluble in the salt solution, and must be soluble in water. The stabilizer polymers generally have an intrinsic viscosity in 1M NaNO3 of from about 0.1-10 dl/g, preferably from about 0.5-7.0 dl/g and more preferably from about 2.0-6.0 dl/g at 30xc2x0 C.
Preferred stabilizers are polyacrylic acid, poly(meth)acrylic acid, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) and copolymers of 2-acrylamido-2-methyl-1-propanesulfonic acid and an anionic comonomer selected from acrylic acid and methacrylic acid.
The stabilizer polymers are prepared using conventional solution polymerization techniques, are prepared in water-in-oil emulsion form or are prepared in accordance with the dispersion polymerization techniques described herein. The choice of a particular stabilizer polymer will be based upon the particular polymer being produced, the particular salts contains in the salt solution, and the other reaction conditions to which the dispersion is subjected during the formation of the polymer.
Preferably from about 0.1 to about 5 percent by weight, more preferably from about 0.25 to about 1.5 percent and still more preferably, from about 0.4 to about 1.25 percent by weight of stabilizer, based on the weight of the total dispersion or finished product, is utilized.
Polymer dispersions prepared in the absence of the stabilizer component result in paste like slurries indicating that a stable dispersion did not form. The paste like products generally thickened within a relatively short period of time into a mass that could not be pumped or handled within the general applications in which polymers of this type are employed.
The remainder of the dispersion consists of an aqueous solution comprising from about 2 to about 40 weight percent based on the total weight of the dispersion of a water-soluble salt selected from the group consisting of ammonium, alkali metal and alkaline earth metal halides, sulfates, and phosphates.
The salt is important in that the polymer produced in such aqueous media will be rendered insoluble on formation, and polymerization will accordingly produce particles of water-soluble polymer when suitable agitation is provided. The selection of the particular salt to be utilized is dependent upon the particular polymer to be produced, and the stabilizer to be employed. The selection of salt, and the amount of salt present should be made such that the polymer being produced will be insoluble in the salt solution. Particularly useful salts include a mixture of ammonium sulfate and sodium sulfate in such quantity to saturate the aqueous solution. While sodium sulfate may be utilized alone, we have found that it alters the precipitation process during polymerization. Salts containing di- or trivalent anions are preferred because of their reduced solubility in water as compared to for example alkali, alkaline earth, or ammonium halide salts, although monovalent anion salts may be employed in certain circumstances. The use of salts containing di- or trivalent anions generally results in polymer dispersions having lower percentages of salt materials as compared to salts containing monovalent anions.
The particular salt to be utilized is determined by preparing a saturated solution of the salt or salts, and determining the solubility of the desired stabilizer and the desired polymer. Preferably from about 5 to about 30, more preferably from about 5 to about 25 and still more preferably from about 8 to about 20 weight percent based on the weight of the dispersion of the salt is utilized. When using higher quantities of monomer less salt will be required.
In addition to the above, other ingredients may be employed in making the polymer dispersions of the present invention. These additional ingredients may include chelating agents designed to remove metallic impurities from interfering with the activity of the free radical catalyst employed, chain transfer agents to regulate molecular weight, nucleating agents, and codispersant materials. Nucleating agents when utilized generally encompass a small amount of the same polymer to be produced. Thus if a polymer containing 70 mole percent acrylic acid (or its water-soluble salts) and 30 percent acrylamide are to be produced, a nucleating agent or xe2x80x9cseedxe2x80x9d of the same or similar polymer composition may be utilized. Generally up to about 10 weight percent, preferably about 0.1 to about 5, more preferably from about 0.5 to about 4 and still more preferably from about 0.75 to about 2 weight percent of a nucleating agent is used based on the polymer contains in the dispersion is utilized.
Codispersant materials to be utilized include dispersants from the classes consisting of water-soluble sugars polyethylene glycols having a molecular weight of from about 2000 to about 50,000, and other polyhydric alcohol type materials. Amines and polyamines having from 2-12 carbon atoms are often times also useful as codispersant materials, but, must be used with caution because they may also act as chain transfer agents during polymerization. The function of a codispersant is to act as a colloidal stabilizer during the early stages of polymerization. The use of codispersant materials is optional, and not required to obtain the polymer dispersions of the invention. When utilized, the codispersant is present at a level of up to about 10, preferably from about 0.1-4 and more preferably from about 0.2-2 weight percent based on the dispersion.
The total amount of water-soluble polymer prepared from the anionic and the nonionic water-soluble monomers in the dispersion may vary from about 5 to about 50 percent by weight of the total weight of the dispersion, and preferably from about 10 to about 40 percent by weight of the dispersion. Most preferably the dispersion contains from about 15 to about 30 percent by weight of the polymer prepared from the nonionic and anionic water-soluble monomers.
Polymerization reactions described herein are initiated by any means which results in generation of a suitable free-radical. Thermally derived radicals, in which the radical species results from thermal, homolytic dissociation of an azo, peroxide, hydroperoxide and perester compound are preferred. Especially preferred initiators are azo compounds including 2,2xe2x80x2-azobis(2-amidinopropane)dihydrochloride, 2,2xe2x80x2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2xe2x80x2-azobis(isobutyronitrile) (AIBN), 2,2xe2x80x2-azobis(2,4-dimethylvaleronitrile) (AIVN), and the like.
The monomers may be mixed together with the water, salt and stabilizer prior to polymerization, or alternatively, one or both monomers may be added stepwise during polymerization in order to obtain proper incorporation of the monomers into the resultant dispersion polymer. Polymerizations of this invention may be run at temperatures ranging from xe2x88x9210xc2x0 C. to as high as the boiling point of the monomers employed. Preferably, the dispersion polymerization is conducted at from xe2x88x9210xc2x0 C. to about 80xc2x0 C. More preferably, polymerization is conducted at from about 30xc2x0 C. to about 45xc2x0 C.
The dispersion polymers of this invention are prepared at a pH of about 3 to about 8. After polymerization the pH of the dispersion may be adjusted to any desired value as long as the polymer remains insoluble to maintain the dispersed nature. Preferably, polymerization is conducted under inert atmosphere with sufficient agitation to maintain the dispersion.
The dispersion polymers of the instant invention typically have bulk solution viscosities of less than about 25,000 cps at 25xc2x0 C. (Brookfield), more preferably less than 5,000 cps and still more preferably less than about 2,000 cps. At these viscosities, the polymer dispersions are easily handled in conventional polymerization equipment.
The dispersion polymers of this invention typically have molecular weights ranging from about 50,000 up to the aqueous solubility limit of the polymer. Preferably, the dispersions have a molecular weight of from about 1,000,000 to about 50 million.
In a preferred aspect of this invention, the stabilizer has a concentration from about 0.25 to about 2 weight percent based on the weight of the total dispersion and an intrinsic viscosity in 1M NaNO3 of from about 0.5-7.0 dl/g.
In another preferred aspect of this invention, the stabilizer is polyacrylic acid; poly(2-acrylamido-2-methyl-1-propanesulfonic acid); an anionic water-soluble copolymer formed by free radical polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid with acrylic acid, wherein the copolymer comprises from about 3 to about 60 weight percent 2-acrylamido-2-methyl-1-propanesulfonic acid and from about 97 to about 40 weight percent acrylic acid; or an anionic water-soluble copolymer formed by free radical polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid with methacrylic acid, wherein the copolymer comprises from about 11 to about 95.5 weight percent 2-acrylamido-2-methyl-1-propanesulfonic acid and from about 89 to about 4.5 weight percent methacrylic acid.
In a more preferred aspect of this invention, the water-soluble polymer is poly (acrylic acid/acrylamide) having a weight ratio of 7:93 for acrylic acid to acrylamide and the stabilizer is poly (2-acrylamido-2-methyl-1-propanesulfonic acid/acrylic acid) having a weight ratio of 13:87 2-acrylamido-2-methyl-1-propanesulfonic acid: acrylic acid.
In another more preferred aspect of this invention, the water-soluble polymer is poly (acrylic acid/acrylamide) having a weight ratio of 7:93 for acrylic acid to acrylamide and the stabilizer is poly (2-acrylamido-2-methyl-1-propanesulfonic acid/acrylic acid) having a weight ratio of 51:49 2-acrylamido-2-methyl-1-propanesulfonic acid: methacrylic acid.
In another more preferred aspect of this invention, the water-soluble polymer is poly (acrylic acid/acrylamide) having a weight ratio of 30:70 for acrylic acid to acrylamide and the stabilizer is poly (2-acrylamido-2-methyl-4-propanesulfonic acid/methacrylic acid) having a weight ratio of 84.7:15.3 2-acrylamido-2-methyl-1-propanesulfonic acid: methacrylic acid.
In a more preferred aspect of this invention, the water-soluble polymer is poly (acrylic acid/acrylamide)-having-a weight-ratio of 30.70 for-acrylic acid to acrylamide and the stabilizer is poly (2-acrylamido-2-methyl-1-propanesulfonic acid/methacrylic acid) having a weight ratio of 90.6:9.4 2-acrylamido-2-methyl-1-propanesulfonic acid: methacrylic acid.
In another more preferred aspect of this invention, from about 0.02 lbs polymer/ton to about 20 lbs polymer/ton, preferably from about 1 lbs polymer/ton to about 15 lbs polymer/ton and more preferably, from about 1 lbs polymer/ton to about 4 lbs polymer/ton of the the high molecular weight water-soluble dispersion polymer is added to the papermaking furnish.
xe2x80x9cPounds polymer/tonxe2x80x9d means pounds of actual polymer per 2000 pounds of solids present in slurry. The abbreviation for pounds of actual polymer per 2000 pounds of solids present in slurry is xe2x80x9clbs polymer/tonxe2x80x9d.
In another more preferred aspect of this invention, a microparticle is added to the pulp.
xe2x80x9cMicroparticlesxe2x80x9d means highly charged materials that improve flocculation when used together with natural and synthetic macromolecules. They constitute a class of retention and drainage chemicals defined primarily by their submicron size. A three dimensional structure, an ionic surface, and a submicron size are the general requirements for effective microparticles. xe2x80x9cMicroparticlesxe2x80x9d encompass a broad set of chemistries including polysilicate microgel, structured silicas, colloidal alumina, polymers, and the like.
Microparticle programs enhance the performance of current retention programs and optimize wet end chemistry, paper quality and paper machine efficiency. Microparticles are not designed to be used as a sole treatment. Rather, they are used in combination with other wet end additives to, improve retention and drainage on the paper machine. Commonly used microparticles include:
i) copolymers of acrylic acid and acrylamide;
ii) bentonite and other clays;
iii) dispersed silica based materials; and
iv) naphthalene sulfonate/formaldehyde condensate polymers.
Copolymers of acrylic acid and acrylamide useful as microparticles include: a representative copolymer of acrylic acid and acrylamide is Nalco(copyright) 8677 PLUS, available from Nalco Chemical Company, Naperville, Ill., USA. Other copolymers of acrylic acid and acrylamide are described in U.S. Pat. No. 5,098,520, incorporated herein by reference.
Bentonites useful as the microparticle for this process include: any of the materials commercially referred to as bentonites or as bentonite-type clays, i.e., anionic swelling clays such as sepialite, attapulgite and montmorillonite. In addition, bentonites described in U.S. Pat. No. 4,305,781 are suitable. A preferred bentonite is a hydrated suspension of powdered bentonite in water. Powdered bentonite is available as Nalbrite(trademark), from Nalco Chemical Company.
Representative dispersed silicas have an average particle size of from about 1 to about 100 nanometers (nm), preferably from about 2 to about 25 nm, and more preferably from about 2 to about 15 nm. This dispersed silica, may be in the form of colloidal, silicic acid, silica sols, fumed silica, agglomerated silicic acid, silica gels, precipitated silicas, and all materials described in Patent Cooperation Treaty Patent Application No. PCT/US98/19339, so long as the particle size or ultimate particle size is within the above ranges. Dispersed silica in water with a typical particle size of 4 nm is available as Nalco(copyright) 8671, from Nalco Chemical Company. Another type of dispersed silica, is a borosilicate in water; available as Nalco(copyright) 8692, from Nalco Chemical Company.
Representative naphthalene sulfonate/formaldehyde condensate polymers useful as microparticles are available as Nalco(copyright) 8678 from Nalco Chemical Company.
The amount of microparticle added is from about 0.05 to about 5.0, preferably from about 1.5 to about 4.5 and more preferably about 2 to about 4.5 pounds microparticle/ton.
xe2x80x9cPounds microparticle/tonxe2x80x9d means pounds of actual microparticle per 2000 pounds of solids present in slurry. The abbreviation for pounds of actual microparticle per 2000 pounds of solids present in slurry is xe2x80x9clbs microparticle/tonxe2x80x9d.
The microparticle is added to the papermaking furnish either before or after the dispersion polymer is added to the furnish. The choice of whether to add the microparticle before or after the polymer can be made by a person of ordinary skill in the art based on the requirements and specifications of the papermaking furnish.
In another preferred aspect of this invention, a coagulant is added to the furnish prior to the addition of the anionic or nonionic dispersion polymer.
In another preferred aspect,the coagulant is a water-soluble cationic polymer.
In another preferred aspect the water-soluble cationic polymer is epichlorohydrin-dimethylamine or polydiallyldimethylarnmonium chloride.
In another preferred aspect, the coagulant is selected from alum or polyaluminum chlorides.
In another preferred aspect, the coagulant is a cationic starch.